The filling I am using in the video is leeks and ricotta, but you are welcome to improvise. Here are a few tips on creating a good filling:

Use cooked ingredients

Cook the filling completely before using (otherwise the steam will get trapped in the ravioli and the dough will get soggy)

Taste the filling before using and adjust salt and acidity. Filling should be very flavorful.

Filling should be thick and dense. Soupy purees don't work for pasta fillings.

Leek Ricotta Filling

2 cups whole-milk ricotta (Calabro is a good brand)

1 large leek, white and pale green parts only
2 Tbsp butter

Olive oil as needed

Salt and pepper to taste

Find a 10-12 inch cast iron skillet to use as a weight. If none is available, stack a few Calculus books and place into a plastic shopping bag so that you don’t mess them up. Place a triple layer of paper towels on a rimmed baking sheet (12x17 of whatever size will fit a square piece of paper towel). Spread ricotta on paper towels into a shape that is slightly smaller than your cast iron skillet (or books). Cover with a triple layer of paper towels and a piece of plastic wrap or foil. Place the weight on top and let sit at room temperature for 1 hour.

Cut the leeks in half lengthwise, rinse the grit between the layers. Cut crosswise into 1/4 inch wide slices. Place in a bowl of cold water, agitate with your hands, and let sit for a minute to allow sand to settle. Carefully remove with a slotted spoon without disturbing the sand on the bottom. Melt butter in a 10 inch skillet over medium heat. Add leeks and a generous pinch of salt. Cook stirring occasionally until tender and all the liquid evaporates. It's ok for the leeks to get golden, but not brown. Add olive oil as necessary to keep them well lubricated. Take off heat and cool. Combine with ricotta, season to taste with salt and pepper. Chill for at least 2 hours and up to 3 days before using.

Herb Cream Sauce

Put 1/2 cup heavy cream into a 2 cup pyrex measuring cup. Add 2 Tbsp finely chopped fresh herbs (any combination of chives, basil, parsley, mint). Puree with an immersion blender until cream foams up and gets thick (it should feel like slightly over-whipped cream). Season to taste with salt and lemon juice. Can be made a few hours ahead and stored chilled.